Friends, this is a bittersweet moment for me. This is the last post for some time, until I decide to pick up the threads again. Thanks to all of you for making this a wonderful journey for the last 5 years. This was a simple puzzle and many of you sent me correct answers. Here are the first 5 to respond, each of whom have been sent an Amazon gift coupon. Gourav Sharma – 6.45 pm Oct 29 Suman Saraf – 6.49 pm Oct 29 Deepak Kumar Singh – 6.56 pm Oct 29 Kartik Aggarwal – 11.38 pm Oct 29 Abdul Hakeem – 6.09 am Oct 30 I am copying the answer from one of the respondents. Thanks again to my daughter, A ..read more

This puzzle was originally posted in April 1969 by Martin Gardner, and is a classic even today. Thanks for an overwhelming response to the puzzle and my apologies for the delayed post as I was traveling on work outside the country over the last two weeks. Many people sent correct answers – this includes Suman Saraf, Kumz91 (sorry do not know the full name), Naresh Kumra, Mohit Rana, Delhi Scrabble (again, do not know the real name), Gaurav Sharma, Viv423 (do not know the name again), Rishikant, G, Rowan an Pratik Poddar. And sorry, if I have missed out anyone. Well done all. Please note that e ..read more

Its been nearly 5 years since I started posting puzzles, and today is the 200th puzzle. I have thoroughly enjoyed posting puzzles, hearing from so many of you, especially some amazing children over the years. But its time for a break now – not sure if this is a few weeks or longer. So, enjoy the last puzzle – first 5 correct answers get a gift from my side! This puzzle was given to be last week by my younger daughter, who got this in her class (grade V) in a math quiz. Its a simple puzzle, where you need to find your way from the entry to the GOAL without taking any left turn. Puzzle #200: Fi ..read more

This was a difficult puzzle, even though it was seemingly simple. I did get attempts from a few people, but the only correct answer came from Gaurav S (founder of d8i.ai) – thanks and well done Gaurav! The site from where I took this puzzle had many  correct answers, most of these were largely a bit of trial and error. One solution, which I modified a little, is as follows: For each of the output node, if the desired input for firing is X, Y, Z (each of these being either a 0 or a 1), design the weights as: -1 if the input should be 0 1 if the input should have been 1 When you do this ..read more

Every time you pick up a Martin Gardner book, there is still always a wonderful puzzle that one has not seen. This one is an original Martin Gardner puzzle, i.e. this is his own creation, published in the book The Colossal Book of Short Puzzles and Problems. Please look at the figure above. This is a desk calendar made out of two cubes that Martin Gardner saw at the Grand Central station in New York in a shop. Each day from 01-31 can be indicated by the two cubes so that their front faces give the required date. Each face can have a unique digit from 0 through 9. What are the four digits that ..read more

This was a relatively simple puzzle, and I got correct answers from Suman Saraf, Mohit Khare, Vishal Poddar and Pratik Poddar – thank you all! The answer is 1024. An easy way to think about this is to create a 10 digit binary number, where each digit represents one of the friends each. 0 means the friend is not coming, and 1 means that the friend is coming. Note that there can be 2^10 combinations here, and each of them represents a distinct combination of friends coming. Hope you all enjoyed the puzzle!   ..read more

Thanks to my INSEAD friend Sergio Werner for pointing me to a new source of puzzles and science articles – http://www.quantamagazine.org. I am reproducing a puzzle posted there by Pradeep Mutalik. Neural networks and deep learning are the buzzwords of the day. Here is a puzzle that shows the basic mechanics of a neural network. We’re going to create a simple network that converts binary numbers to decimal numbers. Imagine a network with just two layers — an input layer consisting of three units and an output layer with seven units. Each unit in the first layer connects to each unit in the seco ..read more

Apologies for not posting anything in the last few weeks, was traveling on work or vacation through this period. This was not a very difficult puzzle, but I got correct answers only from the two people who solve every puzzle – Pratik Poddar and Suman Saraf – thank you and well done! (a) Color the  board like a chessboard, i.e. alternate black and whites. Notice that a bug on a white square can only travel to a black and vice versa. Since there are a total of 81 squares, there will be one more white square (or one more black square). Hence when the bugs move, there will correspondingly be ..read more

This is a very common puzzle, many variants of which one would have. I still like it as the answer is very elegant, and teaches children a methodology. This one is from Mathematical Circles (Russian Experience) by Dmitri Fomin, Sergey Genkin and Ilia Itenberg. A person has 10 friends. Over several days he invites some of them to a dinner party so that the company never repeats itself i.e. the exactly the same set of people cannot repeat itself on any of the days. He may, however, not invite anyone on one of the days. For how many days can he continue to invite people without repetition? As alw ..read more

I received many correct answers. These included  Suman Saraf, Vishal Poddar and Anand Sanghi. Thank you and well done! This puzzle can be solved using the concept of “invariants” and parity. Note that for any numbers a and b (a > b), numbers a and b get replaced by c. If a and b are both odd or both even, then c will be even. If one of them is odd and the other is even, then c will be odd. Essentially, in all the cases, if (a+b) is odd, c will be odd; If (a+b) is even, then c will be even. Therefore, parity of a+b is an invariant. For the final set of numbers to be zero (which is even ..read more